Atherosclerosis is the deposition of fatty plaques on the luminal surface of arteries, which in turn causes narrowing of the cross-sectional area of the artery. Ultimately, this deposition blocks blood flow distal to the lesion causing ischemic damage to the tissues supplied by the artery. Atherosclerosis of the arteries, coronary or peripheral, is a pervasive disease. For example, coronary artery atherosclerosis disease (CAD) is the most common, serious, chronic, life-threatening illness in the United States, affecting more than 11 million persons. The social and economic costs of atherosclerosis vastly exceed that of most other diseases. Narrowing of the coronary artery lumen causes destruction of heart muscle resulting first in angina, followed by myocardial infarction and finally death. There are over 1.5 million myocardial infarctions in the United States each year, and six hundred thousand (or 40%) of those patients suffer an acute myocardial infarction and more than three hundred thousand of those patients die before reaching the hospital (Harrison's Principles of Internal Medicine, 14th Edition, 1998). Narrowing of the peripheral arteries is debilitating and can severely affect the quality of life of afflicted patients.
A number of percutaneous intravascular procedures have been developed for treating stenotic atherosclerotic regions of a patient's vasculature to restore adequate blood flow. The most successful of these treatments is percutaneous transluminal angioplasty (PTA). In PTA, a catheter, having an expansible distal end usually in the form of an inflatable balloon is inserted into a peripheral artery and threaded through the arterial system into the blocked artery and is positioned in the blood vessel at the stenotic site. The balloon is then inflated to flatten the obstructing fatty plaque and dilate the vessel, thereby restoring adequate blood flow beyond the diseased region. Other procedures for opening stenotic regions include directional arthrectomy, rotational arthrectomy, laser angioplasty, stenting, and the like. While these procedures have gained wide acceptance (either alone or in combination, such as PTA in combination with stenting), they continue to suffer from significant disadvantages. A particularly common disadvantage with PTA and other known procedures for opening stenotic regions is the frequent occurrence of restenosis.
Restenosis refers to the re-narrowing of an artery after an initially successful angioplasty. Restenosis afflicts approximately up to 50% of all angioplasty patients and is the result of injury to the blood vessel wall during the lumen opening angioplasty procedure. In some patients, the injury initiates a repair response that is characterized by smooth muscle cell proliferation referred to as “hyperplasia” in the region traumatized by the angioplasty. Acutely, restenosis involves recoil and shrinkage of the vessel, which are followed by proliferation of medial smooth muscle cells. This proliferation of smooth muscle cells re-narrows the lumen that was opened by the angioplasty within a few weeks to a few months, thereby necessitating a repeat PTA or other procedure to alleviate the restenosis. As many as 50% of the patients who are treated by PT A require a repeat procedure within six months to correct restenosis.
Narrowing of the arteries can occur in vessels other than the coronary arteries, including, but not limited to, the aortoiliac, infrainguinal, distal profunda femoris, distal popliteal, tibial, subclavian, mesenteric, carotid, and renal arteries. Peripheral artery atherosclerosis disease (“PAD”, also known as peripheral arterial occlusive disease) commonly occurs in arteries in the extremities (feet, hands, legs, and arms). Rates of PAD appear to vary with age, with an increasing incidence of PAD in older individuals. Data from the National Hospital Discharge Survey estimate that every year, 55,000 men and 44,000 women have a first-listed diagnosis of chronic PAD and 60,000 men and 50,000 women have a first-listed diagnosis of acute PAD. Ninety-one percent of the acute PAD cases involved the lower extremity. The prevalence of comorbid CAD in patients with PAD can exceed 50%. In addition, there is an increased prevalence of cerebrovascular disease among patients with PAD.
A number of different techniques have been used to overcome the problem of restenosis, including treatment of patients with various pharmacological agents or mechanically holding the artery open with a stent or synthetic vascular graft (Harrison's Principles of Internal Medicine, 14th Edition, 1998). Of the various procedures used to overcome restenosis, stents have proven to be the most effective. Stents are metal scaffolds that are permanently implanted in the diseased vessel segment to hold the lumen open and improve blood flow. Placement of a stent in the affected arterial segment thus prevents recoil and subsequent closing of the artery.
There are broadly two types of stents: self-expanding stents and balloon expandable stents. Stents are typically formed from malleable metals, such as 300 series stainless steel, or from resilient metals, such as super-elastic and shape memory alloys, e.g., Nitinol™ alloys, spring stainless steels, and the like. They can also, however, be formed from non-metal materials such as non-degradable or biodegradable polymers or from bioresorbable materials such as levorotatory polylactic acid (L-PLA), polyglycolic acid (PGA) or other materials such as those described in U.S. Pat. No. 6,660,827.
A variety of stent geometries are known in the art, including, without limitation, slotted tube type stents, coiled wire stents and helical stents. Stents are also classified into two general categories based on their mode of deployment. The first type of stent is expandable upon application of a controlled force, such as the inflation of the balloon portion of a dilatation catheter that upon inflation of the balloon or other expansion methods expands the compressed stent to a larger, fixed diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metal or super-elastic alloy such as nickel-titanium (NiTi) alloys that automatically expands or springs from a compressed state to an expanded shape that it remembers.
Exemplary stents are described in U.S. Pat. No. 4,553,545 to Maass et al.; U.S. Pat. Nos. 4,733,665 and 4,739,762 to Palmaz; U.S. Pat. Nos. 4,800,882 and 5,282,824 to Gianturco; U.S. Pat. Nos. 4,856,516, 4,913,141, 5,116,365 and 5,135,536 to Hillstead; U.S. Pat. Nos. 4,649,922, 4,886,062, 4,969,458 and 5,133,732 to Wiktor; U.S. Pat. No. 5,019,090 to Pinchuk; U.S. Pat. No. 5,102,417 to Palmaz and Schatz; U.S. Pat. No. 5,104,404 to Wolff; U.S. Pat. No. 5,161,547 to Tower; U.S. Pat. No. 5,383,892 to Cardon et al.; U.S. Pat. No. 5,449,373, 5,733,303, 5,843,120, 5,972,018, 6,443,982, and 6,461,381 to Israel et al.; U.S. Pat. Nos. 5,292,331, 5,674,278, 5,879,382 and 6,344,053 to Boneau et al.; U.S. Pat. Nos. 5,421,955, 5,514,154, 5,603,721, 5,728,158, and 5,735,893 to Lau; U.S. Pat. No. 5,810,872 to Kanesaka et al.; U.S. Pat. No. 5,925,061 to Ogi et al.; U.S. Pat. No. 5,800,456 to Maeda et al.; U.S. Pat. No. 6,117,165 to Becker; U.S. Pat. No. 6,358,274 to Thompson; U.S. Pat. No. 6,395,020 to Ley et al.; U.S. Pat. Nos. 6,042,597 and 6,488,703 to Kveen et al.; and U.S. Pat. No. 6,821,292 to Pazienza et al., which are all incorporated by reference herein.
Stents are usually delivered in a compressed condition to the target site and then, deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. The delivery system used to implant or deploy at the stent target site in the diseased vessel using a delivery system that comprises a catheter that carries the stent and a control system that allows the stent to be deployed from the catheter into the vessel.
A common method for using such a system to deliver a stent is to advance the catheter into the body of a patient, by directing the catheter distal end percutaneously through an incision and along a body passage until the stent is located within the desired site. The term “desired site” refers to the location in the patient's body currently selected for treatment by a health care professional. After the stent is deployed at the desired site, it will tend to resiliently expand to press outward on the body passage.
Like many catheter systems, a stent delivery system is often used with a flexible guidewire. The guidewire is often metal, and is slidably inserted along the desired body passage. The catheter system is then advanced over the guidewire by “back-loading” or inserting the proximal end of the guidewire into a distal guidewire port leading to a guidewire lumen defined by the catheter system.
Many catheter systems define guidewire lumens that extend along the entire length or almost all of the catheter. These catheter systems are described as “over-the-wire” catheters, in that the guidewires resides inside a catheter lumen throughout the length of the catheter. Over-the-wire catheter systems provide several advantages, including improved trackability, preventing prolapse of the guidewire, the ability to flush the guidewire lumen while the catheter is in the patient, and the capability of easily removing and exchanging the guidewire while retaining the catheter in a desired position in the patient.
In some circumstances it may be desirable to provide a “rapid-exchange” catheter system, which offers the ability to easily remove and exchange the catheter while retaining the guidewire in a desired position within the patient. Rapid exchange catheters are disclosed in U.S. Pat. Nos. 5,380,283 and 5,334,147 to Johnson; U.S. Pat. No. 5,531,690 to Solar; U.S. Pat. No. 5,690,644 to Yurek et al.; U.S. Pat. No. 6,613,075 to Healy et al.; and U.S. Re. Pat. No. 36,104 to Solar.
Rapid-exchange dilatation catheters are capable of advancement into the vascular system of a patient along a pre-positioned guidewire, for balloon angioplasty or a similar procedure. The guidewire occupies a catheter lumen extending only through a distal portion of the catheter. With respect to the remaining proximal catheter portion, the guidewire exits the internal catheter lumen through a proximal guidewire port, and extends in parallel along the outside of the catheter proximal portion. Of course, the entire catheter and guidewire assembly is typically contained within the lumen of a guiding catheter, which retains a majority of the catheter and guidewire effective lengths together.
Because a majority of the guidewire is outside the catheter shaft, it may be manually held in place as the catheter is removed. Moreover, because the distal catheter guidewire lumen is shorter than the guidewire length that remains outside the patient, the catheter may be removed while also holding the guidewire, until the guidewire may be grasped at a point distal of the catheter. Completing a catheter exchange simply requires reversing the removal process. This rapid exchange technique enables a single physician to exchange balloon catheters, without requiring guidewire extension to temporarily double the guidewire length.
Stent delivery systems must ideally possess certain characteristics. For example, the stent delivery system should preferably protect the stent from damage or deformation during delivery. It is further desirable that the stent delivery system be flexible and able to push through and traverse as many different anatomical arrangements and stenosis configurations as possible. In addition, the stent delivery system should provide for high visibility under fluoroscopy. Often the stent delivery system will be used in conjunction with an outer guiding catheter, which surrounds and guides the stent delivery system to the desired site. The visibility of the stent delivery system on a fluoroscope may be affected by the size of the lumen through which radiopaque contrast fluid is injected. This fluid is generally injected through the annular space between the guiding catheter and the stent delivery system. The visibility can, therefore, preferably be increased by further reducing the outer diameter of the stent delivery system.
Moreover, the stent delivery system should preferably have a positive mechanism for retaining the stent on the catheter prior to deployment and then releasing and deploying the stent at the desired site. Thus, a delivery system for implanting a self-expanding stent may include an inner catheter or tube upon which the compressed or collapsed stent is mounted and an outer restraining sleeve or sheath that is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel or accurately positioned at a damaged site, the outer sheath is moved in relation to the inner tube to “uncover” the compressed stent, allowing the stent to assume its expanded condition. Some delivery systems utilize a “push-pull” type technique in which the outer sheath is retracted while attempting to retain the inner lumen stationary. The delivery system may also use an actuating wire that is attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the stent, the inner lumen remains stationary, preventing the stent from moving axially within the body vessel. Many different type of delivery systems have been developed for delivering self-expanding stents, but most require a retractable outer sleeve or sheath.
Because of the narrowness of the human vasculature self-expanding stents, generally, are retained in a highly compressed state within the sheath. As a result of the compressive forces necessary to compress the stent to a small diameter within the sheath or sleeve relatively large forces are required to retract the sheath from the stent. Currently, stent delivery systems utilize hand held devices with pivoting levers to provide the necessary forces to retract the sheath from the stent, i.e., deploy the stent.
In addition to overcoming the sheath retraction problem, a delivery system for self-expanding stents must desirably provide variable speed delivery. Preferably, the delivery system should allow the self-expanding stent to be deployed slowly at first to allow the stent to be accurately positioned at a target site within the vasculature. Once positioned and impinged against the inner vessel wall, it is desirable to provide for more rapid deployment to maintain the position and to increase the speed of the overall procedure. As more of the stent impinges against the wall of the body lumen, the speed of deployment can continue to increase because there is more stent contacting the wall and resisting movement of the stent from its originally deployed position and, therefore, less risk of the stent movement. Hence, there is a need for a delivery system that provides a delivery or deployment speed for self-expanding stents that continues to increase along the length of the stent from a relatively low initial deployment speed to a relatively fast deployment speed as the final portion of the stent is released from the sleeve or sheath.
Some attempts have been made to produce devices that can be operated with a single hand so as to allow a physician to use the free hand to control the movement of the delivery catheter. While generally allowing the user to maintain hand position, these devices have typically not provided a variable rate of deployment of the stent. Two-handed devices have been developed to provide some variable speed capabilities, but these devices generally require the user to alter their hand positions to obtain the variable or differing speeds, which is not desirable as it can lead to inaccurate placement of the stent. These multi-speed devices have used a screw-type mechanism to retract the sheath slowly and then, a sliding mechanism to retract the sheath more quickly. Switching between the two retraction mechanisms requires the user to change hand positions during the deployment of the stent.
Hence, there remains a need for an improved variable speed stent delivery system. Preferably, such a system would allow a user to vary the speed of stent deployment or sheath retraction without requiring a change of hand positions. Additionally, such a delivery system preferably is configured to overcome friction between the sheath and compressed stent in a relatively smooth or fluid manner to facilitate accurate positioning of the stent within a body lumen.